1. Field of the Invention
High frequency "roll-off" and lack of d.c. response limit the capability of audio tape recorders for recording digital data. High frequency roll-off limits the maximum frequency that can be recorded and causes loss of high harmonics in the recorded information. Pulses become rounded and there is a degradation of noise immunity.
2. Description of the Prior Art
In the digital recording technique Return-to-Zero (RZ), a one is represented by a pulse during a cell time and a zero by no pulse (A cell time is the time allocated to each bit and is the reciprocal of the frequency or bit rate.) The RZ recording system requires a good low frequency response to prevent bias drift in the read circuitry during long strings of zeros and also requires a synchronized clock signal to indicate the cell times.
Another technique is Non-Return-to-Zero (NRZ). The NRZ recording technique is one in which a flux transition (induced by a current change in the write head coil) from one polarity to another indicates a change in the data. That is, consecutive ones or zeros have no transition between the individual bits. A transition occurs when the data changes from a digital one to a digital zero or from a digital zero to a digital one. For randomly occurring data, the transitions tend to be more evenly distributed than in RZ recordings, but a good low frequency because of the occasional occurrence of long strings of ones or zeros.
Another technique is the Non-Return-to-Zero Inverted (NRZI). The NRZI technique records a transition for a one but not for a zero (or for a zero but not for a one). When both are used together on two concurrent tracks, timing and error correction are provided.
The energy distribution using the NRZI technique has a spectral density similar to that of NRZ. Both NRZ and NRZI require clock pulses to define the cell times and are sensitive to jitter, i.e., perturbations in timing caused by speed variations.
Another recording technique is the Manchester code or bi-phase recording. In the bi-phase recording technique, the polarity (direction) of a transition during a cell time indicates whether the recorded data is one or zero. For example, a positive transition during a cell time represents a one and a negative transition, a zero. An intercell transition, i.e., a transition at the cell boundary, is required when two bits of the same value are recorded in succession so that the transition during the cell time can be made in the required direction. Having a transition in each bit cell eliminates the need for a clock track and very low frequency response is not required. The spectral energy of bi-phase recording is concentrated at approximately 80% of the bit rate at which the data is recorded.
Another technique for recording digital data is delay modulation (DM) which records a one by a transition in either direction during a bit cell for digital one and no transition for a digital zero unless it is followed by another digital zero, in which case a transition is recorded at the intercell boundary between the two adjacent zero bit cells. Since there is a transition in at least every other cell in the bi-phase technique, the low frequency response is not critical. The lack of a transition during a zero cell when followed by a one reduces the high frequency required so that the spectral energy of a DM signal is concentrated at a frequency lower than the data bit rate, usually about 40% of the bit rate.
Another technique for recording data is pulse length modulation (PLM) or pulse width modulation (PWM) in which the data cell is divided into approximately three equal durations. A pulse extending over the first two thirds of a data cells records a one (or zero) and a pulse extending over the first one third of the data cell records a zero (or one). Data in a PLM system can be read by starting with the leading edge of a pulse a counter which counts at a rapid rate in relation to the cell time until the trailing edge of the written pulse is detected. The counter then counts in the opposite direction. If the count value returns to zero before the occurrence of the transition at the next cell boundary, one binary value is considered to have been read and if the cell boundary edge of the other cell is read before the count value returns to zero, then the other binary value is considered to have been read. The PLM technique, like bi-phase and DM, is self-clocking, i.e., no separate clock track is required. The PLM technique is especially suited for audio recordings using simple read circuitry because the direction of the transition at the pulse edges need not be detected. It has the disadvantage of being susceptible to drop out, i.e., failure to read a transition.
Frequency modulation (FM) is another recording technique in which a transition occurs at each intercell boundary with a one recorded by an intracell transition and a zero, by no intracell transition. The FM technique is self-clocking and permits a high bit packing density. It is, however, susceptible to drop out errors.
The circuit and method of this invention identifies the start of a bit cell by a relatively narrow pulse between cells. A binary digit of one value is recorded by a transition during the cell time and the other binary value, by no transition during cell time. It has the advantage of being useful in inexpensive (low bandwidth) tape recorders because signal distortions are not critical and reasonable speed changes can be tolerated; it is self-clocking, and single drop-out errors can be detected and corrected.